Glycinate compounds, as are obtained by carboxymethylation of the dimethylaminopropylamides of fatty acids, have been known for a long time. Such compounds which are known under the name “amidopropylbetaines” are typically used as surfactants (see e.g., Parfümerie und Kosmetik, 1996, 77(4), 244-248).
Known compounds of this substance class are derived, for example, from coconut fatty acids, hydrogenated coconut fatty acids or coconut fatty acid fractions (e.g., cocoamidopropylbetaine). They are used widely, e.g., as secondary surfactants in cosmetic preparations (e.g., shower gels, shampoos, liquid soaps) and in dishwashing detergents.
A characteristic feature of prior art amidopropylbetaines is their surface-active behaviour, i.e., the property to reduce the surface tension of aqueous systems. This property is caused by the molecular structure, namely by the hydrophobic, long-chain fatty acid radical on one hand, and the hydrophilic, zwitterionic dimethylammonium glycinate group on the other hand. The current fatty acid radicals comprise, in particular, those which occur in natural fats. More particularly, the fatty acid radicals of amidopropylbetaines include coconut fat or hydrogenated coconut fat, namely saturated and unsaturated C8-C18 fatty acids. Amidopropylbetaines of caproic acid (C6) have also been described.
Many zwitterionic substances, such as, for example, amine oxides, proline, ectoin or trimethylglycine are so-called cosmotropes, i.e., they increase the structure or order of aqueous systems. Many of these substances are known to be osmolytes, i.e., they control the water content of cells and thus have a stabilizing effect.
A disadvantageous effect of many surfactants is the irritation of skin and eyes. Although the irritation potential of the amidopropyldimethyl glycinates is lower than, for example, that of the alkyl sulphates or alkyl ether sulphates, the amidopropyldimethyl glycinates are, however, not completely free from possible irritative effects.
As such, there is a need to provide compounds which, despite having the zwitterionic, hydrophilic head group, do not have a cell-damaging effect like surfactants according to the prior art. That is, compounds are needed that contain a zwitterionic, hydrophilic head group, yet have no surface-active structures and thus preferably do not irritate skin and eyes. There is also a need to prepare these zwitterionic compounds using processes which permit large-scale industrial production.